Report on Maldi-Tof Ms on Bacterial Identification and Beta-Lactamase Detection from Positive Blood Culture

HTI 5615 Medical Microbiology Practical Report on Application of MALDI-TOF MS on bacterial identification and Beta-lactamase detection from positive blood culture

Question 1

What is major limitation of using MALDI Sepsityper extraction method for direct identification of bacteria from positive blood cultures?

1. The rate of obtaining accurate species ID from polymicrobial cultures is ranged from 50% to 61.9% while from monomicrobial is ranged from 80.7% to 81.8%. Therefore, the discrimination in polymicrobial samples is poor and identification might be inadequately addressed if the sample contains more than one microbe.
2. Recently, MALDI Biotyper’s database contains about 5600 spectra covering 2000 different species. Thus, it is unable to identify bacterial species that do not exist in the database. Example, poor representation in database for some Gram positive organism, yeast and anaerobes is encountered due to presence of bacterial capsule or insufficient protein signal even added 70% formic acid prior to adding matrix.
3. Some species-level ID such as Bacillus anthracia, Salmonella typhi and Shigella dysenteriae require a separate Security Relevant library since species-level ID could not be obtained from ordinary library, it might increase the costing.
4. Some closely related species could not be differentiated.  Such as Shigella species and Escherichia coli, Streptococcus mitis group and Streptococcus pneumoniae etc.
5. The existing commercial libraries inadequately differentiate all the clinically relevant Mycobacterium species such as Mycobacterium tuberculosis complex.
6. The length of incubation of the fungal cultures significantly influenced the MALDI-TOF MS profile and the fungal database does not performing well for fungi isolated from solid media, only about half isolates could be identified. Thus, it has a technical difficulty and poor reliability on fungal identification.
7. The Reagent cost for Sepsityper is expensive (US $15) when comparing with the in-house one (US $1). However, the accuracy on genus-level and species-level identification does not demonstrate significant increment.
8. More efficient software might be encouraging.

If no reliable bacteria ID is obtained by using this method, what is your contingency plan in order to identify the bacteria at the earliest possible time?

If a reliable bacteria ID cannot be obtained by using MALDI Sepsityper extraction method,  my contingency plan is incubating the sample again on blood agar for 3 hours in an 35℃ CO2 chamber, then pick the colonies for MALDI-TOF MS.